Performance engine builders can’t afford to take chances with questionable valves because they have to withstand high rpm and temperatures. Likewise, rebuilders who overhaul high dollar heavy-duty diesel and industrial engines want to minimize their comeback risks by making sure the valves they use are either new or have been carefully inspected and reconditioned to meet OEM specifications. Yet many rebuilders who do passenger car and light truck engines continue to reclaim and reuse a high percentage of old valves.
In the case of the latter, the primary motivation is obviously cost. It’s cheaper to reclaim and refinish the old valves out of an engine than it is to replace them. In a high competitive and price-sensitive market, saving a buck or more a valve adds up. But what are the risks?
No rebuilder wants an engine to come back because of a valve related problem. Warranty claims can kill your profit margin as well as your reputation.
Every effort needs to be made to make sure reclaimed valves are indeed reusable, and have been properly inspected and reconditioned before they go back into an engine. You also have to make sure they are the correct valves for the application, as some engines run hotter than others and require a higher grade of stainless steel (such as 21-4N) or Stellite or Inconel coated exhaust valves. You also have to make sure that any valve related issues in an engine have been identified and repaired so a repeat valve failure doesn’t occur.
Replacing a broken valve, for example, won’t fix the engine if the underlying cause is misalignment between the valve guide and seat. Unless the misalignment is corrected, the new valve will fail prematurely as flexing causes it to fatigue and eventually break.
Replacing a burned valve won’t fix a compression problem if the underlying cause is a hot spot in the cylinder head. If the cause of the hot spot isn’t identified and eliminated, the new valve will run hot and burn, too.
Replacing worn valve guides by installing new guides or guide liners, or reaming out the guides and installing new valves with oversize stems won’t fix an oil consumption problem if the guide wear is the result of excessive side loading due to rocker arm misalignment. Unless the valve stem height is corrected, the guide repairs won’t last.
Any valve-related problems in an engine should therefore be identified, analyzed and corrected before any valves or guides or seats are reconditioned or replaced. That’s the only way to prevent repeat failures and expensive comebacks. Broken or burned valves as well as worn or loose guides, cracked or loose seats and similar valvetrain damage is often the result of a chain reaction of events. One problem leads to another and eventually a valve fails.
One valve supplier we interviewed for this article said engine builders will often reuse old valves without paying much attention to the condition of the keeper grooves in the stem. If the grooves are worn or damaged, it may result in a valve failure if the valve is reused. His advice? “If in doubt, toss it out.”
To reduce valve related problems, you should do the following:
1) Analyze the amount of wear as well as wear patterns in the head and valvetrain components when the head is disassembled. A careful inspection should reveal any abnormal conditions or wear patterns that would indicate additional problems.
2) Inspect each and every component in the valvetrain and head so all worn or damaged parts can be identified and replaced or reconditioned.
3) Keep a close watch over production quality so the parts that are being reconditioned are done correctly.
4) Pay attention to specifications, critical dimensions and rocker arm geometry to ensure proper reassembly.
Valves Don’t Last Forever
As durable as most engine valves are, they won’t last forever, particularly exhaust valves. Intake valves have the advantage of being constantly cooled by the incoming air/fuel mixture (or air in the case of a diesel). But the exhaust valves live in a constant environment of heat and fire. Because of this, the exhaust valves typically run hundreds of degrees hotter than the intakes, and are therefore far more likely to overheat, burn and fail.
Elevated operating temperatures combined with high rpms and/or lots of miles can cause the valve stems to stretch over time. As the valve stretches, the valvelash closes up and eventually prevents the valve from fully seating. Once this happens, the valve can’t conduct heat away from itself to the seat and into the cylinder head. The valve runs hot and quickly fails.
Valves in performance racing engines are especially vulnerable to this kind of stress. An exhaust valve that may last for years and up to 150,000 miles or more in a typical passenger car engine might only last a season or two in a dirt track or drag racing engine. A set of titanium valves in a NASCAR motor might be replaced after a single race. In a Top Fuel dragster, the valves might be replaced every six to eight runs. That’s not a lot of longevity but the valves in these engines are operating under extreme conditions.
If you’re tearing down a race engine, therefore, some clues to look for that would indicate it’s time to replace the valves would be loss of valve lash (due to valve stretch), unusual or uneven seat wear, scuffing on the valve stems, or wear or chatter marks in the keeper grooves. Valves can also be Magnafluxed or inspected with penetrating dye to check for cracks, and even checked for Rockwell hardness to see if the metal has softened.
The same cautions apply to valves in heavy-duty and industrial engines, too. Many exhaust valves in these engines have some type of hard facing on the exhaust valve face for heat and wear resistance. If the facing is flaking or worn too thin, the valve may not be safe to reuse.
One aftermarket supplier who reconditions heavy-duty diesel and industrial engine valves says they use a multi-step process to inspect valves before they are reconditioned for reuse. Every valve is visually inspected for nicks and damage that could cause stress risers that might eventually lead to valve failure. Next, ultrasound is used to check every valve for internal cracks, followed by an eddy current check to reveal any flaws in the face of the valve. If the thickness of the margin overlay on the valve face does not meet minimum OEM specifications, the valve is discarded.
The overall length of the valve must also meet dimensional criteria, and the hardness of the stem tip is checked to make sure it’s still within specifications. If a valve passes all of these checks, the stem is rechromed to restore the outside diameter to OEM dimensions, and the face is reground to the specified angle. The valve is also shot peened to relieve stress.
With heavy-duty valves, two life cycles is usually considered the limit. Over-the-road trucks can rack up a lot of miles between overhauls, so by the time an engine needs a second overhaul, there’s too much risk of reusing the valves a third time.
The typical passenger car valve, on the other hand, seldom receives the same kind of attention. Passenger car valves are dirt cheap compared to the valves in heavy-duty diesel and industrial engines, so they seldom receive more than a cursory inspection if they are being reclaimed. Obvious problems such as a bent valve stem, mushroomed or grooved valve tip, burned, eroded or cracked valve head, thin valve margin, valve elongation (stretch), or excessive stem wear or galling will result in the valve being tossed into the discard pile. Otherwise, the valves are refinished and reused.
If a valve shows excessive wear on the tip of the stem, the underlying problem is often the wrong installed valve height. This can create back and forth scrubbing across the tip of the stem every time the valve opens and closes.High lift cams and rocker arms can cause this kind of wear if the rockers don’t have roller tips.
Rapid stem wear can also occur if the tips of the valves are ground excessively in an attempt to correct stem height. Grinding through the hardened surface layer leaves the soft metal underneath in direct contact with the rocker arms. Improper shaping of the rocker tips can also increase friction and wear that can result in damage to the stem tips.
Worn Valve Stems
Though severe stem wear and galling is usually obvious to the naked eye, you usually can’t see concentricity problems. Valve stems typically develop uneven wear as a result of side loading. The stem may appear round but at the microscopic level it is out-of-round with one, two, three or more lobes. The uneven wear is often too small to detect with an ordinary two point micrometer. The only way to see this kind of wear is to place the valve in a pair of V-blocks and rotate it 360 degrees while watching a dial indicator.
When lobing is present in a valve stem, it will create runout in the valve head when the valve face is reground. This, in turn, may prevent the valve from sealing and create a compression leak. When you pull vacuum on the cylinder to check the valves, there may be enough leakage to prevent the cylinder from passing the vacuum test.
An out-of-round valve stem can be reconditioned by regrinding on a centerless grinder. This will make the stem round again, but it will also remove the chrome flashing (or nitriding) that was originally on the surface of the stem. This means a reclaimed reground valve must be used with either a bronze liner or guide, or replated to restore the original scuff protection if used with a cast iron guide. If you simply regrind and reuse the valves without replating them, the valves may stick or gall when the engine is first started or if it overheats.
Valve & Guide Replacement Options
Some engine rebuilders will reuse the intake valves but replace the exhaust valves (because the exhaust valves run hotter and are more vulnerable to failure if reused).
If a valve or whole set of valves needs to be replaced, what are your options? You can buy reclaimed and reconditioned valves from various aftermarket suppliers, or you can buy brand new valves from an aftermarket supplier or a dealer. However, some valves may not be available as service parts from the new car dealer because the vehicle manufacturer only wants to sell entire cylinder head assemblies.
With aftermarket suppliers, you usually have a choice of standard or oversized valve stems. Most of the replacement valves that are being sold today are standard size, according to the suppliers we spoke with. Oversized stems were popular back in the days of the high volume production engine rebuilders. But the market has changed drastically from those days, so most engine builders today want valves with standard stems.
One thing to watch out for when buying replacement valves is to make sure the grade of stainless in the new valves has not been “downgraded” to reduce the cost. Many late model Japanese engines are factory-equipped with high quality 21-4N stainless exhaust valves with Inconel facings. Some aftermarket replacement valves for these engines are 21-2N stainless and lack the hard facing. Such valves are probably adequate for everyday light-duty driving, but may not hold up under hard use or in a performance application.
Also, if you are replacing valves, replace the keepers. Don’t reuse worn keepers with new valves. That’s asking for trouble.
On aluminum or cast iron cylinder heads with bronze or cast iron guides, the most common repair for worn guides is to simply knock out the old guides and install new guides. On cast iron heads with integral guides, the repair alternatives for worn guides are to install guide liners, or to ream out the old guides and use new valves with oversized stems. Of course, guide liners or oversized valves are also options for aluminum heads with guides, too. It all comes down to comparing the relative cost of each repair method including parts and labor, and customer preference.
Guide liners are fairly quick, easy and inexpensive to install, so it may be less expensive to use reclaimed valves with guide liners as opposed to reaming out the old guides and using new valves with oversized stems. On the other hand, a customer may want all new stainless steel valves, and may specify a certain type of guide (say bronze rather than cast iron). Consequently, there are numerous ways you can go depending on the application, your customer, what they want and how much they are willing to spend.
The interface between a valve stem and its guide is a critical one for lubrication, cooling and support. Too much clearance between the guides and valve stems will increase oil consumption and decrease valve cooling. This may cause the exhaust valves to runner hotter than normal, accelerate wear and increase the risk of valve burning. Too little clearance, on the other hand, increases the risk of valve stem sticking, galling and seizure.
The surface finish on the valve stem and inside the guide is also critical for valve lubrication and longevity. Valve stems must be relatively smooth to reduce friction, and have surface characteristics that are compatible with the guides. That’s why most valve stems are chrome plated (or have a black nitrited coating in the case of many Japanese engines).
Chrome holds oil and protects the stem from galling. For performance racing valves, the stems may be chromed, or they might have some type of hard carbon Physical Vapor Deposition (PVD) or Plasma Assisted Chemical Vapor Deposition (PACDV) coating to reduce friction and wear.
The finish inside the valve guides is also important because they guides must be smooth but also capable of retaining enough oil to provide adequate lubrication for the valves. When guides are reamed to size, a second honing step is often done to improve the surface finish. Honing helps remove any torn or folded metal left by the reamer and plateaus the surface to reduce friction and improve oil retention.
Article courtesy of ENGINE BUILDER magazine.